LiCoO_2 Epitaxial Film Enabling Precise Analysis of Interfacial Degradations

Interfacial structure evolution and degradation are critical to the electrochemical performance of LiCoO_2 (LCO), the most widely studied and used cathode material in lithium ion batteries. To understand such processes requires precise and quantitative measurements. Herein, we use well-defined epitaxial LCO thin films to reveal the interfacial degradation mechanisms. Through our systematical investigations, we find that surface corrosion is significant after forming the surface phase transition layer, and the cathode electrolyte interphase (CEI) has a double layer structure, an inorganic inner layer containing CoO, LiF, LiOH/Li_2O and Li_xPF_yO_z, and an outmost layer containing Li_2CO_3 and organic carbonaceous components. Furthermore, surface cracks are found to be pronounced due to mechanical failures and chemical etching. This work demonstrates a model material to realize the precise measurements of LCO interfacial degradations, which deepens our understanding on the interfacial degradation mechanisms.